As is known, antennas currently used in radio communication devices, such as USB plug-in dongles, are characterized by nearly omnidirectional radiation diagrams (i.e., have low gains) that severely limit achievable performances in terms of data rate.
In general, a low antenna gain limits coverage range in those scenarios, such as rural areas, where effect of thermal noise is predominant over inter-cell interference.
A larger antenna gain is then beneficial for reducing digital divide because it potentially enables provision of high data rate connections in areas not reached by fixed Digital Subscriber Lines (DSL(s) or xDSL(s)).
Moreover, an advantage of using high gain directional antennas concerns the possibility to perform interference rejection in spatial domain by electronically shaping the radiation diagram of the antenna. This aspect is very important in urban areas characterized by high voice/data traffic where the main limiting factor for the achievable data rate is the inter-cell interference.
Performances of radio communication devices can be improved by providing them with multiple antennas that can be exploited in different ways depending on their characteristics, such as directionality, polarization, relative spacing, etc.
In particular, the simplest way to improve performances of radio communication devices is to use multiple omnidirectional antennas widely spaced apart in order to achieve a certain level of spatial diversity from fading. Signals from the different omnidirectional antennas can be recombined at baseband level by using a number of RF receivers equal to the number of receiving antennas.
A drawback of this solution is that one RF receiver is required for each antenna and thus the number of omnidirectional antennas that can be used in practice is rather limited.
Besides, small dimensions of the current radio communication devices, especially in case of portable radio communication devices such as USB plug-in dongles or mini-hubs, limit even further the number of, and the relative distance among, the antennas that can be integrated together. In particular, an undesired effect caused by a small distance between the antennas is an increase of fading correlation that limits the diversity level that can be achieved and, in case of Multiple Input Multiple Output (MIMO) transmission, makes not possible a parallel spatial multiplexing of multiple data streams.
A solution to the problem of having multiple RF receivers is provided by Applicant's international patent application WO2008064696.
In particular, WO2008064696 discloses a wireless communication system which is equipped with a switched-beam antenna comprising a certain number of directional antenna elements and wherein a sub-set of RF signals received from corresponding antenna elements is selected and combined into a single RF signal that is processed and demodulated in a single processing chain.
In particular, the wireless communication system disclosed by WO2008064696 comprises an RF phasing network for co-phasing the selected RF signals before combining them, and a processor for controlling RF signal combining and phasing in order to obtain a single RF signal having a Radio Performance Indicator (RPI) which satisfies predetermined conditions. The combination of the received RF signals is performed at radiofrequency by using a set of combining weights taken from a predefined set or codebook. According to WO2008064696, the minimum number of RF receivers is just equal to the number of transmitted data streams whatever the number of receiving antennas is.